It is a 2007 sporttrac LTD with a 4.6 V8. When I start the truck in the morining or if it has been sitting for a while it sounds as if I have an exhaust leak as it ticks. If I rev the engine it backfires but after it warms up it works excellent and when you rev the engine it does not backfire. It really does not matter if it cold, warm or damp outside when it does this. Once and a while I will start it and it will not tick but if I shut it off and then restart it it does tick. Please help